Supergravity is a type of quantum theory of elementary particles and their
interactions that is based on the particle symmetry known as supersymmetry
and that naturally includes gravity along with the other fundamental
forces (the electromagnetic force, the weak nuclear force, and the strong
nuclear force).

The electromagnetic and the weak forces are now understood to be different
facets of a single underlying force that is described by the electroweak
theory. Further unification of all four fundamental forces in a single
quantum theory is a major goal of theoretical physics. Gravity, however,
has proved difficult to treat with any quantum theory that describes the
other forces in terms of messenger particles that are exchanged between
interacting particles of matter. General relativity, which relates the
gravitational force to the curvature of space-time, provides a respectable
theory of gravity on a larger scale. To be consistent with general
relativity, gravity at the quantum level must be carried by a particle,
called the graviton, with an intrinsic angular momentum (spin) of 2 units,
unlike the other fundamental forces, whose carriers (e.g., the photon and
the gluon) have a spin of 1.

A particle with the properties of the graviton appears naturally in
certain theories based on supersymmetry--a symmetry that relates fermions
(particles with half-integral values of spin) and bosons (particles with
integral values of spin). In these theories supersymmetry is treated as a
"local" symmetry; in other words, its transformations vary over
space-time, unlike a "global" symmetry, which transforms uniformly over
space-time. Treating supersymmetry in this way relates it to general
relativity, and so gravity is automatically included. Moreover, these
supergravity theories seem to be free from various infinite quantities
that usually arise in quantum theories of gravity. This is due to the
effects of the additional particles that supersymmetry predicts (every
particle must have a supersymmetric partner with the other type of spin).
In the simplest form of supergravity, the only particles that exist are
the graviton with spin 2 and its fermionic partner, the gravitino, with
spin 3/2. (Neither has yet been observed.) More complicated variants also
include particles with spin 1, spin 1/2, and spin 0, all of which are
needed to account for the known particles. These variants, however, also
predict many more particles than are known at present, and it is difficult
to know how to relate the particles in the theory to those that do exist.